Rarefaction

(noun.) a decrease in the density of something; 'a sound wave causes periodic rarefactions in its medium'.

昌西整理

• The pulse of condensation and rarefaction which travels the length of the wire is called a wave, although it bears little or no resemblance to the familiar water wave. 伯莎M.克拉克. 科学通论.
• Sound waves may be said to consist of a series of condensations and rarefactions, and the distance between two consecutive condensations and rarefactions may be defined as the wave length. 伯莎M.克拉克. 科学通论.

校对:拉弗尔斯